1. Field of the Invention
The present invention relates to one-chip microcomputers which can be connected to an external memory.
2. Description of the Prior Art
When address and other data are sent from a one-chip microcomputer to an external memory, the one-chip microcomputer outputs bus data from an output terminal via an output buffer. FIG. 4 is a block diagram showing the buses and signal output unit of a conventional one-chip microcomputer. In the figure, plural address buses 1a-1c, the number of which is equal to the bit count of an address of the one-chip microcomputer, are connected to output terminals 5a-5c through output buffers 2a-2c; 40 is the CPU; 41 the working RAM; and 42 the external memory. CPU 40 is also connected with data line d, clock line CLK, and enable line EN.
FIG. 5 is a timing diagram showing the operation of an address signal output unit of the one-chip microcomputer shown in FIG. 4. In the figure, 14 is the internal clock output from CPU 40; 15 is the enable signal; 16 shows a signal from one of address buses 1a-1c output as an inverse of the previous bus state (i.e., H.fwdarw.L or L.fwdarw.H); 20 is an output signal from one of buffers 2a-2c to which address signal 16 is input; through current 21 flows to output buffers 2a-2c when signals from the output buffers are inversed-e.g., when output signal 20 from output buffer 2a changes from "H" to "L" or from "L" to "H." Further, when reading external memory 42, enable signal 15 returns data signal 22 on data line d while the L state is on, following a duration t2 after the detection of an address signal in output signal 20 from output buffer 2a-2c.
Due to the structure described above, an address bus signal output unit is a conventional one-chip microcomputer generates a large external load capacity when connected to external memory 42, and must thus be driven by an output buffer with large-sized transistors. When plural output buffers change state, plural address signal lines also change state at the same time within the short time period ta, causing a large amount of through current to flow to the output buffers. This through current becomes great as the number of address buses changing state at the same time increases, causing a flow of reverse current in the power line and therefore current fluctuations, which are a cause of malfunctions in one-chip microcomputers.